During the examination of the eye of a patient, measurements are frequently taken, the results of which are documented, and which, therefore, must be reproducible.
Among others, these include corneal curvature, anterior chamber depth, iris diameter (e.g., according to the white-to-white method), and the length of the ocular axis. In order for these measurements to be reproducible, it is particularly important, aside from the measuring accuracy, that during the measuring procedure, the measuring instrument is positioned precisely and reproducibly with regard to the patient's eye.
From U.S. Pat. No. 5,463,430, a keratometer is known, whereby two different marking systems are used for determining the correct measuring distance. At first, a circular reference marking is reflected in the optical path, whereby its apparent diameter should correspond with the average diameter of a patient's cornea. With said marking, the operator determines the alignment and, roughly, the correct distance of the patient's eye to the measuring instrument. Subsequently, a more precise determination of the distance is made in such a way that parts of the index marking system, intended for the actual measuring, are projected onto the cornea as “finite” as well as “infinite.” For the switch between “finite” and “infinite,” a collimator lens is occasionally removed from the optical path of the projection; for the actual measurement, said lens has to be realigned. When the correct distance is set, the measuring process is triggered automatically, which is indicated to the operator through a color change of the circular reference marking.
In U.S. Pat. No. 5,905,562, it is suggested that the axial alignment is determined through projection of an additional measuring marking onto the cornea, subsequent mapping of the resulting image and evaluation through image processing; determination of the measuring distance is carried out the same way as in U.S. Pat. No. 5,463,430. In this example, the measurement is also triggered automatically once correct alignment has been ascertained.
Said solutions exhibit a number of disadvantages. For example, the determination of the measuring distance through the necessary realignment of the collector lens is time-consuming and can, therefore, lead to incorrect measuring results in case of a movement by the patient; furthermore, additional mechanical requirements are necessary.
Experience has also shown that automatic triggering of the measurement in said solutions does not always produce optimal results, in accordance with prior art, since, e.g., the imaging of the eye can be distorted through eye lashes protruding into the optical path of the measuring instrument, insufficiently developed tear film, etc., which may lead to faulty measurements. Said conditions cannot be recognized by the solutions of prior art; therefore, automatic triggering may take place even if unsuitable measuring conditions are present, which, in turn, may lead to incorrect measurements, and which, in principle, are difficult to recognize or even completely unrecognizable as such.